Circuits to delay a signal from DDR-SDRAM memory device including an automatic phase error correction

ABSTRACT

A method for delaying a control signal, includes receiving a clock signal, determining a number of delay elements required to generate a first delay equal to a target amount of the period of the clock signal, receiving a data signal having an edge generated at the same time as an edge of the control signal, determining a fraction number equal to the number of delay elements needed to generate a second delay for the data signal or the control signal to align their edges, divided by the number of cascaded delay elements necessary to provide a delay equal to the target amount of the period of the clock signal, multiplied by the number of delay elements to generate the first delay, and delaying the control signal by the number of cascaded delay elements to realize said first delay altered by the fraction number of delay elements.

RELATED APPLICATIONS

This application is a continuation-in-part of co-pending applicationSer. No. 11/466,311, filed Aug. 22, 2006.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to dual-data-ratedynamic-random-access-memory (DDR-SDRAM) devices. More particularly, thepresent invention relates to circuits to delay the “DQS” signal from aDDR-SDRAM memory device to capture data, the “DQS” signal and the databeing generated simultaneously by the memory device, the circuitsincluding a fine delay tuning capability.

2. The Prior Art

DDR-SDRAM devices can transfer data twice as fast as single-data-rateSDRAM memory devices (SDR-SDRAM). This is because DDR-SDRAM devices cansend and receive signals twice per clock cycle. This feature increasesthe complexity of writing data to and reading data from the DDR-SDRAMdevice since the valid-data window is narrower than in SDR-SDRAMdevices.

Referring now to FIGS. 1A and 1B, a timing diagram illustrates the validdata time windows for SDR-SDRAM devices (FIG. 1A) and DDR-SDRAM devices(FIG. 1B) with relation to the clock timing. From FIG. 1A, it may beseen that there is a single valid data window for each complete cycle ofthe SDR-SDRAM clock. From FIG. 1B, it may be seen that there are twovalid data windows for each DDR-SDRAM clock cycle.

In an application system, for example a microcontroller circuitconnected to DDR-SDRAM devices on a printed circuit board, the signalDQS is a bidirectional control signal transmitted by the DDR-SDRAMdevices during read operations and by the memory controller during writeoperations. The memory controller may be part of a microcontrollerintegrated circuit. For DDR device circuitry optimization, the DQSsignal is provided edge-aligned with data for read operations and shouldbe center-aligned with data for write operations. The DQS signal and itsrelationship to the valid data windows in a typical read operation, isshown in FIG. 3 and the DQS signal and its relationship to the validdata windows in a typical write operation, is shown in FIG. 2.

To write data to DDR-SDRAM devices without increasing the complexity ofthe DDR-SDRAM controller and to guaranty that the signal iscenter-aligned with data, it is possible to use the falling edge of aclock signal running at twice the frequency of the clock that drives theDDR-SDRAM devices. This aspect of operation of a DDR-SDRAM device isshown with reference to FIG. 2, in which waveforms illustrate that, fora write access from a DDR-SDRAM device, the rising and falling edges ofa DDR-SDRAM DQS signal are center aligned with the valid data. TheDDR-SDRAM controller generates signals with such phase relationship.

As also shown in FIG. 3, if delayed with an appropriate time increment,the delayed DQS signal is aligned with the center of the valid datawindow, the DQS signal can be used as a sample and hold signal whichmakes a simple, safe circuitry to capture data from DDR-SDRAM device.

During read operation, the DQS signal is edge-aligned with data, thecontroller delays the DQS signal by a period of time corresponding toabout ¼ of the DDR device clock period to allow the alignment of thedelayed DQS signal with the center of the valid data window. Under thiscondition, the data from the DDR device can be properly sampled becausethe hold/setup time margins are optimal (middle of data valid window,321, 322, 331, 332). Of course, the delay must be stable.

A simple delay circuitry having DQS as its input and formed from a delayline of cascaded basic cell elements such as buffers or inverters doesnot guaranty a stable delay because basic element intrinsic delaydepends on de-rating factors such process, voltage, and temperaturevariations.

BRIEF DESCRIPTION OF THE INVENTION

A method for delaying an input control signal, comprises receiving aninput clock signal; determining a number of cascaded delay elementsrequired to form a first delay equal to a target amount of the period ofthe input clock signal; receiving an input data signal having an edgegenerated with an edge of the input control signal; determining thenumber of cascaded delay elements needed to form a second delay for oneof the input data signal and the input control signal by an amountnecessary to align the edge of the input data signal with the edge ofthe input control signal; and delaying the input control signal by anamount of time equal to the first delay altered by the second delay suchthat the edge of the input control signal is delayed from the edge ofthe input data signal by the target amount.

A circuit according to the present invention for delaying an inputcontrol signal comprises a circuit for receiving an input clock signaland determining a number of cascaded delay elements required to form afirst delay equal to a target amount of the period of the input clocksignal; a circuit for receiving an input data signal having an edgegenerated with an edge of the input control signal and determining thenumber of cascaded delay elements needed to form a second delay for oneof the input data signal and the input control signal by an amountnecessary to align the edge of the input data signal with the edge ofthe input control signal; and a circuit for delaying the input controlsignal by an amount of time equal to the first delay altered by thesecond delay such that the edge of the input control signal is delayedfrom the edge of the input data signal by the target amount.

During read operation, the DQS signal is edge-aligned with data.According to the present invention, the controller must delay the DQSsignal by a theoretical period of time corresponding to ¼ of the DDRdevice clock period. Due to different derating factors, the DQS and DATAsignals are not 100% edge aligned in real life operations and thereforethe delay value must be adjustable around ¼ of the DDR device clockperiod. Under this condition, the data from the DDR device can beproperly sampled because the hold/setup time margins are optimallylocated in the real middle of the data-valid window.

This logic is a kind of delay locked loop acting as master circuitry tocalculate the number of cascaded basic elements required to produce aknown delay and keep it stable with respect to conditions such asprocess variations, voltage, and temperature. The master circuitrydrives a slave delay circuit that applies the required and stable delayto the DQS signal. The master circuitry (DLL) allows determination of astable delay (about ¼ of a clock period) regardless of the deratingfactors (e.g., process, voltage and temperature). The time referenceentered into the master circuitry is a clock signal whose frequency is afraction of the DDR device clock frequency. The DQS and data phasesrelative to the DDR device clock may vary from one printed circuit boardto another due to their different topologies and differences in internalcircuitry topologies of the memory devices. The DQS phase may also varydue to derating factors such voltage drops.

The slave circuitry delays the DQS signal by the stable delay (about ¼DDR clock period). Therefore the output of the slave circuitry can beused as data sampling command. The DLL circuitry determines the numberof basic elements such as buffers or inverters to be cascaded to delaythe DQS signal by a given amount of time (about ¼ of the DDR clock). Thenumber of delay elements may be modified/adjusted on the fly to obtainthe stable delay.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

FIG. 1 is a timing diagram illustrating the valid data window ofSDR-DRAM and DDR-DRAM devices.

FIG. 2 is a timing diagram illustrating a typical DDR-SDRAM writeaccess.

FIG. 3 is a timing diagram illustrating a typical DDR-SDRAM read access.

FIG. 4 is a diagram illustrating a typical application environment inwhich a microcontroller is shown driving a DDR-SDRAM device.

FIG. 5 is a block diagram illustrating a circuit according to thepresent invention for producing a DQS signal having a programmabledelay.

FIG. 6 is a diagram illustrating a typical programmable delay line thatis suitable for use in the present invention

FIG. 7 is a diagram illustrating a N/M multiplier circuit suitable foruse in the present invention.

FIGS. 8A through 8C are timing diagrams showing the locked and twounlocked conditions for the circuit of FIG. 5.

FIG. 9 is a diagram showing another phase detector circuit like that ofFIG. 5 and further including an output signal indicating a lockedcondition.

FIG. 10 is a series of timing diagrams illustrating the several relativetimings between valid data and a DQS signal.

FIG. 11 is a diagram illustrating a DQS-delay circuit according to thepresent invention.

FIG. 12 is a diagram illustrating an example of a ratio calculating andFSM circuit suitable for use in the circuit of FIG. 11.

FIG. 13 is a diagram illustrating another example of a ratio calculatingand FSM circuit suitable for use in the present invention.

FIG. 14 is a diagram illustrating another DQS-delay circuit according tothe present invention.

DETAILED DESCRIPTION OF THE INVENTION

Persons of ordinary skill in the art will realize that the followingdescription of the present invention is illustrative only and not in anyway limiting. Other embodiments of the invention will readily suggestthemselves to such skilled persons.

Referring now to FIG. 4, a block diagram illustrates a typicalenvironment in which the present invention may be employed. FIG. 4 showsa system including a microcontroller 10 connected to a DDR-SDRAM device12 through a DDR memory controller 14. An input clock signal, shown atreference numeral 16, provides a clock reference signal to DQS delaycircuitry 18. The function of DQS delay circuitry 18 is to delay theDQS(0) and DQS(1) signals from DDR-SDRAM device 12 to produce a delayedDQS(0) and a delayed DQS(1) signal to control reading data fromDDR-SDRAM device 12. It is to a DQS delay circuitry 18 that the presentinvention is directed.

As may be seen from an examination of FIG. 4, the delayed DQS(0) anddelayed DQS(1) signals are used to clock D-flip-flops 20 and 22,respectively. D-flip-flops 20 and 22 are used to latch the lower-orderand upper-order data bits read from DDR-SDRAM device 12 and presented tomicroprocessor 10 on read data bus 24. In addition, gated clock 26 andwrite-data logic 28 in DDR memory controller 14 generate the signalsnecessary to write data from microprocessor 10 into DDR-SDRAM device 12.As will be appreciated by persons of ordinary skill in the art,bidirectional buffers 30 and 32 are interposed between DDR-SDRAM device12 and DQS delay circuitry 18 and bi-directional buffer 34 is interposedbetween DDR-SDRAM device 12 and DDR memory controller 14. These buffersare controlled as known in the art to pass data in the proper directionfor read and write operations by conventional circuitry (not shown).

To make the DQS signal delay stable, a programmable delay line is usedand tuned with regard to variations in the derating factor. This tuningis automatically performed by a locked loop circuit (master). Therespective DQS and data phases may also vary from one printed circuitboard to another due to different printed circuit board topologies andalso to internal discrepancies of the DDR memory device circuits,resulting in the necessity to tune the theoretical delay applied to theDQS signal. The phase of the DQS signal may also vary due to de-ratingfactors such internal or external voltage drops. Therefore aprogrammable delay line more complex than a simple delay line is used inthe present invention. Such a programmable delay line employs aprogrammable number of basic delay units as will be disclosed herein.Independent master circuitry is used to keep track of the deratingvariations to select, in real-time, the number of basic delay elementsused in the programmable delay line to provide a given delay for the DQSsignal input. The independent master circuitry is provided with a stabledelay reference and locks on to the stable delay reference using anumber of basic delay cells identical to the programmable delay lineused to delay the DQS signal input. The locked system ensures trackingvariations in the derating factor.

For design convenience and simplicity, the stable time reference enteredinto the master circuitry is the clock signal of the DDR-SDRAM memorycontroller or a clock signal having a frequency which is a sub-multiple(divided by 2, etc.) of the frequency of the memory controller to makethe DQS delay circuitry more simple to design.

The slave circuitry receives the DQS signal as input and delays it bythe stable delay (about ¼ DDR clock period, subject to fine tuning tomatch DQS and data phase variations. Therefore the output of the slavecircuitry driven by the master circuitry can be used as data samplingcommand.

Referring now to FIG. 5, a simplified schematic diagram shows anillustrative example of DQS Delay circuitry 40 that provides theaforementioned features according to the present invention. A simplifiedschematic includes blocks 42, 44, 46, and 48. The reference delay willbe provided by block 42, then master locked loop circuitry 44 willdetermine the number of basic delay elements to cascade to obtain thereference delay. This number of delay elements will be converted inblock 46 to get the final number of basic delay elements to delay theDQS signals by means of slave delay lines 48.

Block 42 allows obtaining a programmable reference delay by employingcircuitry that multiplies the input frequency on line 50 by theprogrammable ratio N/M in multiplier 52, whose output has a frequencyvalue equal to [(N/M)*F_(input)], where f_(input) is the input frequencyon line 50. The output 54 of block 42 is the system clock of the DQSdelay circuitry 40 and will act as a reference signal/delay. For designconvenience and simplicity, the stable time reference entered into themaster circuitry may be the clock signal of the DDR-SDRAM memorycontroller divided by 2, therefore N=1, M=2.

This programmable value allows modification of the optimal data samplingpoint. The theoretical value of the optimal data sampling point is ¼ ofthe DDR-SDRAM clock period, but due to different printed circuit boardson which data and DQS signals are routed with different wire lengthsand/or capacitances, the terminal points of these signals may bedifferently phased. Therefore, the optimal sampling point will benominally about ¼ of the clock period but may end up to be a little bitmore or less. As these conditions can vary from one printed circuitboard to another, it is important to provide the capability to tune thesampling point through the user interface of the DDR-SDRAM controller.

Different methods exist to generate a programmable delay, and the module40 of FIG. 5 is one example. The fractional coefficient multiplier canuse a phase-locked loop (PLL) and two simple clock dividers to get afractional divider as will be shown with reference to FIG. 6.

For the descriptions of next modules, it is assumed that module 42provides an output clock period on signal line 54 being twice the inputclock period provided to the DDR-SDRAM device (i.e. if the DDR-SDRAM isclocked at 100 MHz, the frequency at signal line 54 is 50 MHz). Thiswill make the reference delay stable by keeping it independent of thevariations of duty cycle on signal 50. Therefore, having no capabilityof adjustment on this side, there is a need to get this capability ofadjustment somewhere. The present invention provides this fineadjustment.

Block 44 contains the circuitry that locks on the reference delayprovided by module 42. It allows determination of the number of basicdelay elements of a delay line 56 to obtain a delay which is a fractionof the system clock period.

The number of delay elements determined by block 44 will be a knownfraction of the number of elements required to delay the DQS signal fromDDR-SDRAM devices. The delay line used in module 44 is designed with thesame basic delay elements as the one that will be used in the slavedelay line to delay the DQS signal.

In the following example, the module 44 is designed in such a way thatit locks on half of a system clock period. This leads to a simplifiedcircuit architecture to reach the lock state from initial or reset stateor from lock to lock state (due to a derating factor variation).

As in all locked systems, the architecture comprises a phase detectorcircuit to provide the information necessary to add or remove basicdelay elements in the programmable delay line 56 to match the referencedelay provided by stable clock signal 54.

In the example shown in FIG. 5, the phase detector circuit includesD-flip-flops 58 and 60, delay line elements 62 including a limitednumber of basic delay elements (buffers or an even number of inverters),a NOR gate 64 and an AND gate 66. The circuit is driven by clock input54, and uses the output of delay line 62 and the output of theprogrammable delay line 56 as a feedback clock.

When system reset is asserted on line 68, the D-flip-flops 58 and 60 arecleared, the programmable master delay line provides a feedback clock atthe output of master programmable delay line 56 delayed by a singlebasic delay element because the up/down counter 70 is set accordinglyfrom the outputs of NOR gate 64 and an AND gate 66.

After de-assertion of system reset on line 68, the D-flip-flops 58 and60 start sampling logical “0” (the low portion of the waveform at theoutput of master programmable delay line 56). When the outputs of bothD-flip-flops are cleared, the 2-input NOR gate 64 provides a logical 1at the “UP” input of up/down counter 70 to indicate that the phasedetector 44 is unlocked and requires more basic delay elements to beincluded in the master programmable delay line to reach the lock state.The 2-input AND gate 66 drives the “DOWN” input of the up/down counter70 with a logical “0” to indicate that there is no need to remove delayelements in the programmable delay line 56. An example of this state isshown in FIG. 8A.

The up/down counter 70 modifies its output to instruct masterprogrammable delay line 56 to add more delay. The programmable delayline increases its internal delay accordingly by selecting 1 more basicdelay. The phase detector module 44 is still in its unlocked state.

If the delay becomes greater than the reference delay provided by theclock period of system clock at its output 54, both D-flip-flops 58 and60 sample a logical “1.” The 2-inputs NOR gate 64 returns logical “0” tothe “UP” input of up/down counter 70 and the 2-input AND gate 66provides a logical “1” to the “DOWN” input of up/down counter 70. Underthese conditions, up/down counter 70 modifies the value provided on itsoutput to instruct the master programmable delay line 56 to remove onebasic delay element. The master programmable delay line decreases itsinternal delay accordingly. The phase detector 44 is still in itsunlocked phase. An example of this state is shown in FIG. 8C.

When the programmable delay line 56 delays the system clock on signalline 54 by half the system clock period (locked state), D-flip-flop 58samples a logical “1” whereas D-flip-flop 60 samples a logical “0.” Thisdifference of sampled values is possible due to the presence of delayline 62 in the path of the data input of D-flip-flop 58.

Delay line 62 allows locating the falling edge of the delayed feedbackclock at the output of delay line 62 to a time after the rising edge ofsystem clock on line 54 and locating the falling edge of the feedbackclock at the input delay line 62 prior to the rising edge of the systemclock on line 54. In this case both NOR gate 64 and AND gate 66 providelogical “0” to the “UP” and “DOWN” inputs of up/down counter 70. Theoutput of up/down counter 70 does not change, indicating that the phaseerror provided by the phase detector is zero and the phase detector 44is locked. An example of this state is shown in FIG. 8B. If thepropagation delay of phase detector 44 is greater than the higher valueof the metastable period among D-flip-flops 58 and 60 plus the minimumdelay in the programmable delay line 62, the phase detector will stay ina locked state without metastable behavior of D-flip-flops 58 and 60.Metastable states will occur in transient phases.

The delay line 62 can be designed with basic delay elements suchclassical inverters or buffers. There is no need for more complex delayelements as will be disclosed with reference to the master programmabledelay line 56.

The propagation delay between the input of delay line 62 and its outputmust be greater than a value defined as the sum of the setup and holdtime of the D-flip-flops 58 and 60. This will limit the metastablebehavior on both D-flip-flops for each sampling point. If one of thedelayed signals to the data inputs of D-flip-flops 58 and 60 arrives inthe metastable period of one D-flip-flop, then the other signal cannotbe in the metastable period of the second one.

Persons of ordinary skill in the art will appreciate that there is stilla probability of one of the D-flip-flops sampling data during a setup orhold period. There is no way to avoid this situation but an improvementexists in the definition of the intrinsic delay value (delay line 62) ofphase detector 44.

In its locked state, the phase detector 44 defines a number of basicdelay elements needed to delay the system clock by half the system clockperiod. A main objective of the present invention is to get ¼ of the DQSperiod or ¼ of the DDR-SDRAM device clock period. Therefore a conversionmust be performed and applied to programmable delay line connected toDQS control input signals.

Referring now to FIG. 6, an illustrative programmable delay line circuit80 to use as a programmable delay line such as master programmable delayline 56 in FIG. 5 is shown. The illustrative programmable delay linecircuit 80 in FIG. 6 is shown having a plurality of cascaded unit delayelements 82, 84, 86, 88, 90, and 92. Each unit delay element includes aninverter and a multiplexer. The inverter of each unit delay elementbeing cascaded with the inverter of the next unit delay element and themultiplexer of each unit delay element has one input cascaded with theinverter of the previous unit delay element. Thus unit delay element 82includes inverter 94 and multiplexer 96; unit delay element 84 includesinverter 98 and multiplexer 100; unit delay element 86 includes inverter102 and multiplexer 104; unit delay element 88 includes inverter 106 andmultiplexer 108; unit delay element 90 includes inverter 110 andmultiplexer 112; unit delay element 92 includes inverter 114 andmultiplexer 116. The purpose of inverter 118 is to balance thecapacitive load for each stage of the programmable delay line andtherefore it balances the propagation delay of each stage. An inputbuffer 120 and an output buffer 122 are provided.

Multiplexers 96, 100, 104, 108, 112, and 116 are controlled by switchinginputs S₀, S₁, S₂, S₃, . . . S_((n−1)) and S_((n)), respectively. If theselect of a unit delay element is set to logic zero, its multiplexerselects the inverted output of the multiplexer in the next unit delayelement. If the select of a unit delay element is set to logic one, itsmultiplexer selects the output of its own inverter. Thus, only oneselect input in the programmable delay line circuit 80 need be set tologic one, in which unit delay element the signal is turned around andis directed back down through the chain of multiplexers and ultimatelyto the output buffer 122. Any select input further downstream in thechain that is set to logic one does not affect the operation of theprogrammable delay line circuit 80.

As an example, if the select inputs S₀ S₁ are set to logic zero and theselect input S₂ is set to logic one, the signal will pass through theinput buffer 120, inverters 94, 98 and 102, multiplexers 104, 100 and96, and through output buffer 122. The states of select inputs S₃, . . .S_((n−1)) and S_((n)) will not affect the operation of the circuit.

Referring again to FIG. 5, block 46 functions to convert the data fromthe output of up/down counter 70 to a value that may be used by theslave programmable delay line circuits 130 and 132 in block 48 of thecircuit of FIG. 5. Slave programmable delay line circuits 130 and 132may also be configured as shown in FIG. 6. Module 46 in the circuit ofFIG. 5 performs a converter function and allows to modification of theslave programmable delay line circuits 130 and 132. The Delay LockedLoop comprising phase detector 44 is locked on half the clock period(i.e. the programmable delay line 56 delays the input clock signal online 54 by half the clock period). Thus, using an identical slaveprogrammable delay line to delay the DQS input control signal by ¼ ofthe clock period provided to the DDR-SDRAM device, the number of basicdelay elements to select is ¼ of the value reported by up/down Counter70 because the lock is performed on half period of a clock which isdivided by 2 versus the clock provided to the DDR memory.

Block 46 includes a fractional coefficient multiplier 134, whose inputmay be updated as necessary by the output of up/down counter 70. Itsoutput is presented to D-flip-flop 136 via multiplexer 138. The datalatched in D-flip-flop 136 is used to drive slave programmable delaylines 130 and 132 of block 48. The select input of multiplexer 138 isdriven by the update delay line signal at line 140. As long as theupdate signal is not asserted, the output of D-flip-flop 136 is fed backto its data input through multiplexer 138. When the update signal 140 isasserted, the input of D-flip-flop 136 is driven by the output ofup/down counter 70.

Due to the structure of the programmable delay line 56 as has been shownand described with reference to FIG. 6, the input value to supply to theswitching inputs of the multiplexers in the delay line to select thedelay amount is not a decimal coded value but rather a one-hot value.Therefore to divide the input value by 4, fractional coefficientmultiplier 134 may be configured as a look-up table. The functionalityof fractional coefficient multiplier 134 can be seen as a fractionalcoefficient multiplier on a non-decimal base. Table 1 shows an exampleof look-up table embedded in fractional coefficient multiplier 134.

TABLE 1 Number of selected Number of basic selected elements basicelements in Master in Slave Input Value Delay Line Output Value DelayLine 1000000000000000 1 1000000000000000 1 0100000000000000 21000000000000000 1 0010000000000000 3 1000000000000000 10001000000000000 4 1000000000000000 1 0000100000000000 51000000000000000 1 0000010000000000 6 0100000000000000 20000001000000000 7 0100000000000000 2 0000000100000000 80100000000000000 2 0000000000000001 16 0001000000000000 4

Care must be taken when changing the delay value. The value returned byfractional coefficient multiplier 134 cannot be applied to the slaveprogrammable delay line at any time. It is preferable to apply a newvalue when there is no access being made to data from the DDR-SDRAMdevice. If this value is altered when the memory device is beingaccessed, the value must be held to avoid modifying the DQS delay whenthe DQS signal is in use to avoid the risk of a parasitic pulse whenswitching from one delay to another one in the programmable delay line.At any rate, if accesses are performed without interruption, there is aneed to update the delay to take into account the possible deratingfactor variations. The DDR-SDRAM devices need to periodically interruptthe accesses to be able to refresh their contents. The times of theserefresh cycles are known by the memory controller. This information canbe used to safely enable the update of the slave delay line duringrefresh operations when the DQS signals are not used by the DDR-SDRAMmemory controller and glitches on that line will not matter.

If such a scheme is used, when the memory controller (not shown)instructs the DDR-SDRAM device to perform refresh, it asserts a signalon line 140, thereby refreshing the contents of D-flip-flop 136. As soonas refresh period is finished, the line 140 is de-asserted and themultiplexer 138 re-circulates data to D-flip-flop 136.

Referring now to FIG. 7, an illustrative circuit for multiplier 52 ofFIG. 5 is shown. A N/M multiplier may be formed from a PLL 150 and twoclock dividers 152 and 154. As an example, the PLL 150 can multiply theinput signal by 8, 9, 10, 11, or 12 and the divide the resultingfrequency by 10. The range of frequency on clock line 54 will be within+/−20% of the initial frequency. As a consequence, the delay locked loopmodule 42 will lock on a different reference delay and the user willhave the ability to modify the delay of the DQS signal. The fractionalcoefficient multiplier can be a single value and, in such a case, thedesign is simpler than a PLL. It can be a simple divider by two (DFFwith negated output connected on its data input).

Referring now to FIG. 9, a diagram shows another phase detector circuit160 like that of FIG. 5 but further including an output signalindicating a locked condition of the detector. The elements of the phasedetector circuit 160 of FIG. 9 that correspond to elements of FIG. 5 areidentified by the same reference numerals that are used in the circuitof FIG. 5.

As in the example shown in FIG. 5, the phase detector circuit 44includes D-flip-flops 58 and 60, delay line elements 58 including alimited number of basic delay elements, a NOR gate 64 and an AND gate66. The circuit is driven by clock input 54, and uses the output ofdelay line 62 and the output of the programmable delay line 56 as afeedback clock. In addition to these elements that are shown also inFIG. 5, the phase detector circuit of FIG. 9 includes AND gate 162having the one of its inputs that is driven from the output ofD-flip-flop 58 inverted. The other input driven from the output ofD-flip-flop 60 is not inverted. The output of 162 will be used toinstruct the fine delay tuning that the master circuitry is locked.

When system reset is asserted on line 68, the D-flip-flops 58 and 60 arecleared, the programmable master delay line provides a feedback clock atthe output of master programmable delay line 56 delayed by a singlebasic delay element because the up/down counter 70 is set accordinglyfrom the outputs of NOR gate 64 and an AND gate 66.

After de-assertion of system reset on line 68, the D-flip-flops 58 and60 start sampling logical “0” (the low portion of the waveform at theoutput of master programmable delay line 56). When the outputs of bothD-flip-flops are cleared, the 2-input NOR gate 64 provides a logical 1at the “UP” input of up/down counter 70 to indicate that the phasedetector 160 is unlocked and requires more basic delay elements to beincluded in the master programmable delay line to reach the lock state.The 2-input AND gate 66 drives the “DOWN” input of the up/down counter70 with a logical “0” to indicate that there is no need to remove delayelements in the programmable delay line 56. The output of AND gate 162provides a logical “0” to indicate that the phase detector 160 isunlocked.

If the delay becomes greater than the reference delay provided by theclock period of the system clock, both D-flip-flops 58 and 60 sample alogical “1.” The 2-inputs NOR gate 64 returns logical “0” to the “UP”input of up/down counter 70 and the 2-input AND gate 66 provides alogical “1” to the “DOWN” input of up/down counter 70. Under theseconditions, up/down counter 70 modifies the value provided on its outputto instruct the master programmable delay line 56 to remove one basicdelay element. The master programmable delay line decreases its internaldelay accordingly. The phase detector 160 is still in its unlockedphase.

When the programmable delay line 56 delays the system clock on signalline 54 by half the system clock period (locked state), D-flip-flop 58samples a logical “1” whereas D-flip-flop 60 samples a logical “0”because of the presence of delay line 62 in the path of the data inputof D-flip-flop 58.

Delay line 62 allows locating the falling edge of the delayed feedbackclock at the output of delay line 62 to a time after the rising edge ofsystem clock on line 54 and locating the falling edge of the feedbackclock prior to the rising edge of the system clock on line 54. In thiscase both NOR gate 64 and AND gate 66 provide logical “0” to the “UP”and “DOWN” inputs of up/down counter 70. The output of up/down counter70 does not change, and the output of AND gate 162 presents a logical“1” to indicate that the phase error provided by the phase detector 160is zero and the phase detector 160 is thus locked.

Referring now to FIG. 10, a series of timing diagrams illustrates theseveral relative timings between valid data and a DQS signal. The firstthree traces of FIG. 10 show, respectively, the DDR-SRAM clock, the DQSsignal from the DDR-SRAM device during a read operation, and the idealpositioning of valid data from the read operation as being edge alignedwith the DQS signal. The period of the DQS signal is indicated as timet₁. The fourth trace of FIG. 10 shows the DQS signal optimally delayedby a time ¼t₁.

The fifth trace of FIG. 10 shows a condition where read data is valid ata time interval t₂ prior to the rising edge of the DQS signal. Underthis condition, the DQS signal should optimally delayed by a timet₃=¼t₁−t₂ as shown in the sixth trace, so that the edge of the DQSsignal follows the edge of the valid data by a time equal to ¼t₁.

The seventh trace of FIG. 10 shows a condition where read data is validafter a time interval t₄ following the rising edge of the DQS signal.Under this condition, the DQS signal should optimally delayed by a timet₅=¼t₁+t₄ as shown in the eighth trace, so that the edge of the DQSsignal follows the edge of the valid data by a time equal to ¼t₁.

Referring now to FIG. 11, a diagram illustrates a DQS delay circuit 170according to the principles of the present invention. Like the circuitof FIG. 5, the DQS delay circuit 170 of FIG. 11 uses the system clock onsignal line 172 and a calibration request signal on line 174 to drive acalibration request input in delay-locked loop and sample/hold circuit176 through OR gate 178. The other input of OR gate 178 is driven by anenable-measure signal asserted on enable-measure signal line 180. Thedelay-locked loop and sample/hold circuit 176 outputs a lock signal anda multi-bit ¼ clock delay signal used to drive the slave delay units asdisclosed with reference to FIG. 5.

The enable-measure signal on signal line 180 also drives enablecircuitry 182. When enabled by asserting the enable-measure signal onsignal line 180, enable circuitry 182 examines the state of the locksignal output of delay-locked loop and sample/hold circuit 176 on line184, and the output of D-flip-flop 186 which functions as a phasedetector to detect any delay between the DQS signal and the data fromthe DDR-SDRAM. The measurement of any DQS/data delay difference willonly be performed when the master delay locked loop circuit is in itslocked state.

The clock input of D-flip-flop 186 is driven from the output ofprogrammable slave delay line (0) 188 and the data input of D-flip-flop186 is driven from the output of programmable slave delay line (1) 190.The inputs to both programmable slave delay line (0) 188 andprogrammable slave delay line (1) 190 are driven through identicalmultiplexers 192 and 194. Multiplexer 192 has its select input connectedto ground so that it always passes the data from its “0” input to itsoutput. The data presented to its “0” input is the DQS (0) signal fromthe DDR-SDRAM. Multiplexer 194 has its select input connected to theenable-measure signal line 180, its “0” input to the DQS (1) signal fromthe DDR-SDRAM, and its “1” input to a data bit output (such as thedata[0] bit) from the DDR-SDRAM.

During the measurement process, two memory addresses in the DDR-SDRAMare loaded with a “0” and a “1” respectively and the address bus of theDDR-SDRAM is toggled between those two memory addresses so that theoutput data from the DDR-SDRAM appearing at the “1” input to multiplexer194 is a square wave having the same frequency as the DQS signal. Duringnormal operation of the circuit, multiplexer 194 passes the DQS(1)signal to the input of programmable slave delay line (1) 190. Theenable-measure signal selects the data from the DDR-SDRAM to appear atthe output of multiplexer 194 during the measurement process.Multiplexer 192 is placed in the DQS signal data path simply to preventphase error from being introduced by multiplexer 194 in the data path.Persons of ordinary skill in the art will observe that the multiplexersare not needed in systems that have a single DQS signal.

Programmable slave delay line (1) 190 delays the toggled square-wavedata output signal from the DDR-SDRAM at the output of multiplexer 194,providing a delayed signal at output on line 196 of programmable slavedelay line (1) 190. At the beginning of the measurement process, theselected delay is close to zero, and thus the rising edge of the signalon line 196 coupled to the data input of D-flip-flop 186 occurs prior tothe delayed DQS signal on line 198 at the output of programmable slavedelay line (1) 190 coupled to the clock input of D-flip-flop 186. Theoutput of D-flip-flop 186 will latch a logical “1” and present a clockpulse to increment the counter 200 so long as the state of the locksignal output of delay-locked loop and sample/hold circuit 176 on line184 is locked, the enable measure signal on line 180 is asserted and theend measurement signal at line 202 is unasserted, AND gate 206 drives alogical “1” to the select input of multiplexer 208, which passes thecounter output to programmable slave delay line (1) 190. The output ofAND gate 206 also drives a logical “1” to the select input ofmultiplexer 210, thus passing the ¼ clock delay output of master delaylocked loop circuit 176 through multiplexer 210 to control the delay inprogrammable slave delay line (0) 188.

Thereafter, the circuitry will start the measuring process byincrementally delaying the data signal on line 196 with respect to thedelayed DQS signal that is used to clock D-flip-flop 186. Once the counthas incremented to a value that causes a delay in programmable slavedelay line (1) 190 sufficient to cause the delayed DQS signal drivingthe clock of D-flip-flop 186 to latch a logical “0” at the output ofD-flip-flop 186 and present it to enable circuitry 182 on low_delaysignal line 212, counter 200 stops incrementing. At this point, theratio-calculating and FSM circuit 214 starts performing its calculation.

Referring now to FIG. 12, a diagram illustrates an example of aratio-calculating and FSM circuit 214 suitable for use in the circuit ofFIG. 11 according to the principles of the present invention. Accordingto the present invention, to be able to adjust the theoretical ¼ clockdelay provided by master delay locked loop circuit 176 of FIG. 11 whenthe memory device is powered, the ratio between both delays iscalculated. First, the count representing the amount of delay requiredto align the delayed data on line 196 of FIG. 11 with the delayed DQSsignal on line 198 of FIG. 11 is presented to the subtrahend input 220of subtractor circuit 222 and the count representing the delayed DQSsignal is presented to the minuend input 224 of subtractor circuit 222.The output of subtractor circuit 222 is the difference in delay betweenthe rising edge of the DQS signal and the edge of the data signal. Theoutput of subtractor circuit 222 is a signed value because the risingedge of the DQS signal may be located either before or after the edge ofthe data signal. According to the sign of the difference, carried by theMSB of the output of subtractor circuit 222, the operating adjustmentwill be positive or negative.

The difference between the two delays is compared to the DQS delay onsignal line 224, but first the absolute value is calculated to furtherobtain a ratio. Subtractor circuit 226 and multiplexer 228 togetherfunction as an absolute value circuit in which the difference betweenthe two delays is presented to the subtrahend input of subtractorcircuit 226 and to the “0” input of multiplexer 228, a value of “0”(shown symbolically as ground) is presented to the minuend input ofsubtractor circuit 226, the output of subtractor circuit 226 ispresented to the “1” input of multiplexer 228, and the MSB (sign) bit ofthe difference value is presented to the select input of multiplexer228.

Adders 230, multiplexers 232, AND gates 234, and D-flip-flops 236together function as a multi-bit accumulator, the output of which iscompared in comparator 238 to the count representing the delayed DQSsignal on line 224. When the enable measure signal on line 240 is notasserted, AND gates 234 maintain the accumulator in a “cleared” state.When the enable measure signal on line 240 is asserted, and the endmeasure signal is cleared, the multiplexer 232 selects the output of theadder 230. The set of AND gates 234 is transparent because the enablemeasure signal on line 240 is asserted. Together with comparator 238 andcounter 242, they provide at the output of counter 242 an image of theratio between number of cascaded basic elements to produce the clockperiod and the number of cascaded basic elements to produce a delayequivalent to the delay difference between DQS and a bit of DATA.

On the first rising edge of the system clock on line 240, the set ofD-flip-flops 236 latch the phase difference present at the output of theabsolute value circuit because the reset value of the D-flip-flops is alogical “0.” This first value is compared in comparator 238 with thedelayed DQS signal on line 224 and is, of course, lower in value,leaving the output of comparator 238 at a logical “0” state. Multiplexer232 thus still selects the output of adders 230 as its input. The outputof the D-flip-flops 236 (the phase difference) accumulates at the outputof the D-flip-flops 236 at each rising edge of the system clock. Thecounter 242 increments since its enable input (the output of comparator238) is still at a logical “0.”

When the value at the outputs of the D-flip-flops 236 becomes higherthan (or equal to) the value of the delayed DQS signal on signal line224, the counter is no longer enabled and the accumulator stops becausethe select input of multiplexer 232 now selects the recirculated valueout of the D-flip-flops. The ratio between the values on signal lines220 and 224 is now present at the output of counter on signal lines 244.

As an example of the operation of the circuit of FIG. 12, if the ¼ DQSperiod requires 36 basic delay elements in the programmable delay lineand the delay between the rising edge of the data[0] to the rising edgeof the DQS signal requires 9 basic delay elements, the ratio is 1:4. Theaccumulator starts at zero through 9, 18, 27, and 36, and the respectivevalues of the signal at the output of the comparator 238 are 0, 0, 0,and 1. Counter 242 will count up to four. In operation of the presentinvention, values may range between about 8 and 2. Lower values mayindicate that the difference in delay between the rising edges of thedata and the DQS signal are critical and may be unacceptable for correctoperation of the DDR-SDRAM memory device. Higher values may indicatethere is nothing significant to adjust.

To make the adjustment according to the formula:ADJdelay=TheoreticalDelay+/−(ratio×theoretical delay),the ratio is 1/N where N is the value in the counter at the end of themeasurement, it is required that N be an integer. To simplify the delaycorrection circuitry of the present invention, the counter value onsignal lines 244 is reduced to a power of 2 to enable the use of simplecircuitry for performing the division. This reduction is performed byLUT 246, which simply converts the counter output to static decimalvalues 2, 4, and 8. Table 2 shows an exemplary truth table for LUT 246.

TABLE 2 Decimal Input Decimal Output Ratio 2 0 1/2 3 0 1/2 4 1 1/4 5 11/4 6 1 1/4 7 2 1/8 8 2 1/8 9, 10, . . . 3 No Adjustment

Dividing by powers of 2 employs simple circuitry to right-shift thedata. The output of LUT 246 drives the select input of multiplexer 248to select the appropriate bits of the data from signal line 224 thatrepresent the appropriate bit shifted data which represents ½, ¼, ⅛ ofthe value carried on signal 224. The value is still positive, but may beadded or removed depending on the sign of the phase difference (the MSB)carried on signal line 250. The MSB signal on line 250 is latched bymultiplexer 252 and a DFF 254. This is mandatory because when the systemis not in the measure mode, the MSB of subtractor circuit 222 does nothave the same meaning and that value must be available at the end ofmeasure period. The output of DFF 254 holds this value when the systemis not in the measure mode.

If there is a need to remove delay from the theoretical optimal delay incases where the rising edge of the data occurs prior to the rising edgeof the DQS signal, the value to align the delayed data with the delayedDQS signal requires a delay lower than a ¼ clock delay and the signal isnegative (MSB set). The choice of addition or subtraction isaccomplished by using subtractor circuit 256 and multiplexer 258. Theoutput of multiplexer 248 is presented to the subtrahend input ofsubtractor circuit 256 and “0” (shown symbolically as ground) ispresented to the minuend input of subtractor circuit 256. The sign bit(MSB value) is used to drive the select input of multiplexer 254. Theoutput of the multiplexer 258 provides either the negative value of theratio multiplied by the DQS delay when the MSB is set or the positivevalue when the MSB is not set. Finally, the properly signed ratiomultiplied by the DQS delay is added to the ¼ clock period DQS delay inadder 260. During operation, if the number of basic delay elementsrequired to delay the DQS signal by ¼ clock period changes, the numberof delay elements needed to adjust the DQS/data difference willautomatically be updated without a need to calculate the required numberof delay elements.

The adjusted delay will be provided to the slave programmable delaylines 188 and 190 of FIG. 11 through multiplexers 208 and 212 of FIG.11. When the end of measure signal is asserted, AND gate 206 selects the“0” inputs of multiplexers 208 and 212.

Because it is likely that a delay difference may occur between eachindividual data bit on the data bus, persons of ordinary skill in theart will realize that the circuits of FIGS. 11 and 12 can separatelycalculate the delay for each data bit on the bus and calculate anaverage of the ratios to use to adjust the theoretical optimal delay.Further, the adjustment of DQS(1) signal may be accomplished byselecting among the data read on the DQS(1) half of the DDS-SDRAM readcycle and comparing it with the DQS(1) signal in the same manner astaught herein for the DQS(0 signal.

The accuracy of the fine delay adjustment may be enhanced according tothe present invention. In the embodiments shown in FIGS. 11 and 12, thedelay ratio between ¼ clock period and the delay between DQS/DATA isfirst calculated, and then the fine delay tuning is based on this ratio.For example, ¼ clock period requires 100 cascaded basic elements, i.e.@100 Mhz: 2.5 ns requires 100 elements, if DQS and DATA have a delaydifference of 650 ps, 25 basic elements will be required to correct thisdifference. In the embodiments disclosed so far herein, if thetemperature decreases (and the intrinsic delay of basic element alsodecreases), the number of elements required to produce a ¼ clock perioddelay would be, for example, 200 elements and the fine adjustmentapplied would be 50 basic elements to obtain the same adjustment ratio(100/25=200/50).

Using this scheme, the ¼ clock period remains identical in terms of time(2.5 ns) whereas the DQS/DATA difference has a great probability ofbeing reduced from 650 ps to 325 ps (illustrative values only).Therefore the number of delay elements is 25 rather than 50. To obtainnon-linearity in the ratio the circuitry of FIG. 12 may be simplified.Such a simplified scheme is shown in circuit 260 of FIG. 13.

Referring now to FIG. 13, it is seen that, in circuit 270, thedifference between number of delay elements carried on lines 272 and 274is calculated by subtractor 276 and stored into DFF storage element 278by re-circulating its output by means of set of multiplexers 280 whenthe enable measure signal is de-asserted at the select input ofmultiplexers 280. When the enable measure signal is asserted, the DFFssample the difference. This difference in the number of elements will beadded to the value on line 274 in adder 282. The number carried on theoutput of DFFs 278 is a signed value, therefore a simple adder 282 isused to add or subtract from the value on line 274.

Assuming the same values used in the previous example, if the delaydifference decreases from 650 ps to 325 ps, the number of cascaded basicelements involved in the adjustment will not be 50 but 25 because thenumber of basic elements is involved in the adjustment rather than aconstant ratio that increases or decreases the number of elements.

Moreover, for cell place-and-route improvement, the architecture may befurther modified by removing the multiplexers 192 and 194 and performingchecks on data read from the memory devices as will now be describedwith referenced to FIG. 14. This architecture provides exactly the samefunctionality as the architecture embedding multiplexers as was shown inFIG. 11. This modification can employ either a ratio adjustment or theadjustment described with reference to FIG. 13.

Referring now to FIG. 14, it may be seen that the circuit issubstantially similar to the circuit of FIG. 11. The programmable slavedelay lines 188 and 190 receive the DQS 0 and DQS 1 signals. Specificdata is written in the memory device as described with referenced toFIG. 11. The DQS signals are delayed with the theoretical ¼ clockperiod, and the data is sampled and checked. If the data matches thespecific expected value, the DQS is delayed by 1 less basic delayelement than the theoretical value. The data read from the memory deviceis checked again, if it matches the specific expected value, the delayis decreased again by one basic element (the module FSM 214 setting theselect input of the programmable delay accordingly) and so on.

As soon as the read data does not match the specific data, the moduleFSM 214 stores this minimum number of delay elements in a first registerformed from DFF and multiplexer elements. Then the DQS delay isinitialized again with the theoretical value and the process is repeatedexcept that the programmable delay is increased instead of beingdecreased. As soon as the read data does not match the specific patternthe FSM module 214 stores the second number of basic elements in asecond set of registers formed from DFF and multiplexer elements. Atthis point, the output of both registers are sent to a subtractor,subtracting the value in the first register from the value in the secondregister. Then a selected of the subtractor output is kept. The LSB isunused to provide a divide by 2. This divide by 2 value corresponds tothe number of basic elements to be cascaded in the programmable delayline to get the optimal DQS sampling point due to the DQS/DATA phasedifference. To keep track of this DQS/DATA difference further duringusage of the DDR controller (i.e. during functional operation of thememory device), the location of the optimal sampling point with thetheoretical sampling point carried on output of master DLL output isknown. To get the difference, the divided by 2 value and the theoretical¼ clock period are the inputs of the circuit of FIG. 13 on lines 272 and274. The delay to adjust (second delay) is automatically calculated froma real DQS/data difference, and there is no need for firmware/softwareto enter a value in calculation/fsm module 214.

While embodiments and applications of this invention have been shown anddescribed, it would be apparent to those skilled in the art that manymore modifications than mentioned above are possible without departingfrom the inventive concepts herein. The invention, therefore, is not tobe restricted except in the spirit of the appended claims.

1. A method for delaying an input control signal, comprising: receivingan input clock signal; determining a number of cascaded delay elementsrequired to generate a first delay equal to a target amount of theperiod of the input clock signal; receiving an input data signal havingan edge that was generated at the same time as an edge of the inputcontrol signal; determining a fraction number equal to the number ofcascaded delay elements needed to generate a second delay for one of theinput data signal and the input control signal equal to an amount oftime necessary to align the edge of the input data signal with the edgeof the input control signal, divided by the number of cascaded delayelements necessary to provide a delay equal to the target amount of theperiod of the input clock signal; delaying the input control signal byan amount of time realized by the number of cascaded delay elements torealize the first delay altered by the fraction number of delayelements.
 2. The method of claim 1 wherein the target amount of theinput clock period one quarter of the system clock period.
 3. The methodof claim 1, wherein the input control signal is received from a memorydevice and the number of delay elements needed to delay the inputcontrol signal is updated during a period of inactivity of the memorydevice.
 4. The method of claim 1, wherein the input control signal is aread data strobe received from a double data rate synchronous dynamicrandom access (DDR-SDRAM) memory device.
 5. The method of claim 4,wherein determining the fraction number is performed periodically atleast once during a normal mode of operation of the DDR-SRAM memorydevice.
 6. The method of claim 4, wherein determining the number ofcascaded delay elements required to generate a first delay is performedby checking the data value read from DDR-SRAM memory device.
 7. Acircuit for delaying an input control signal, comprising: means forreceiving an input clock signal; means for determining a number ofcascaded delay elements required to form a first delay equal to a targetamount of the period of the input clock signal; means for receiving aninput data signal having an edge that was generated at the same time asan edge of the input control signal; means for determining the number ofcascaded delay elements needed to form a second delay for one of theinput data signal and the input control signal by an amount of timenecessary to align the edge of the input data signal with the edge ofthe input control signal; and means for delaying the input controlsignal by an amount of time realized by a number of cascaded delayelements required to create the said first delay altered by the numberof delay elements determined at the time of the second delaycalculation.
 8. The circuit of claim 7, wherein the input control signalis a read data strobe received from a double data rate synchronousdynamic random access (DDR-SDRAM) memory device.
 9. The circuit of claim7, wherein the means for determining the second delay operates at leastonce during a normal mode of operation of the DDR-SRAM memory device.10. The circuit of claim 7, wherein the means for determining the numberof cascaded delay elements required to generate a first delay isperformed by checking the data value read from the DDR-SRAM memorydevice.